I had mine (both rear) changed earlier this year, ordered some SNR's from eurocarparts, they currently have them at £64.80 (from the 1.9).
My mechanic changed them and said that the caps had rusted to hell, hence why water has got in and ruined them, so check your caps, and get some more if need be before the weather gets worse. (the 147 caps fit, got mine from a scrappy for £2.50 each).
I think mine lasted for a while from the first rumbling, but the ride will get worse as time goes on, so I'd rather get it done sooner rather than later.
Just remembered, he said the worst one (passenger side, where the cap was a lot worse) was a nightmare to get off, probably because I left it so long to get it changed.